A parabolic reflector is a constituent element of a microwave frequency parabolic antenna. Taking advantage of the unique property of that dish shaped reflective surface, wherein RF energy incident at any location on the surface is reflected to the parabola's focal point, the antenna's feed component is located at the surface's focal point. As a consequence, the more diffuse essentially spatially displaced RF fields propagating through space and incident at displaced positions on the reflector are concentrated or focused to a single point, thereby producing a more intense RF field at that point. That advantage permits intelligible reception of weaker RF signals than otherwise could be detected. For the foregoing reason and other reasons well known to those skilled in the art, the parabolic antenna is widely used in communications systems, including those found in space vehicles.
In space vehicle application those antennas are "deployable". That is, the antenna is constructed of a structure that may be collapsed or folded up into a package of small volume, suitable for stowage in the limited space available on board a space craft. It may then be expanded to a much larger size structure, following launch and orbital positioning of that space craft.
RF Deployable parabolic reflectors in space vehicle application typically employ a reflective cloth-like fabric as the reflective surface typically constructed of a cross hatch of wires welded together at the intersections or knitted gold plated molybdenum wire. The reflective fabric is light weight and pliant in nature, so it may be compacted as part of the stowed package. When the reflector is deployed, the fabric is stretched out taut by the associated supports to form a parabolic curved surface.
One such prior deployable reflector incorporates an umbrella-like foldable structure, which, like an umbrella, unfolds radially outwardly extending spokes of longitudinally curved geometry that supports the pliant cloth-like metal mesh reflective surface and stretches the reflective fabric into the required parabolic shape. The ribs are attached at an end with hinges to and are distributed about the periphery of a central joint or hub. For deployment, the ribs are swung radially outward from the central hub, carrying the attached reflective surface and rendering that surface taut. Deployable reflectors of the foregoing kind are known and call to mind the Fltsatcom Transmit antenna manufactured by TRW, Inc., the TDRSS SA antenna manufactured by Harris; and the PAMS reflectors, a TRW, Inc. development.
The profile accuracy of the reflector is dependent upon the wavelength of the RF signal being transmitted or received. Typically the profile is controlled in shape by the number of shaped ribs used in the arrangement. The greater the number of ribs, the smaller the arcuate spacing between those ribs, and the closer the surface geometry of the tautened reflective material supported by the ribs conforms to a paraboloid.
At some frequencies, the extending curved ribs alone are sufficient to adequately shape the reflective surface in the deployed state. At higher frequencies where the wavelength is much shorter, additional shape control devices, referred to as catenaries, are often strung between adjacent ribs at spaced intervals from the central hub. Those catenaries ensure that the portion of the reflective surface located in the interstices between adjacent ribs are held to the proper parabolic shape.
To fit in the stowed envelope for launch, the reflectors fold at the hub, much like the ordinary umbrella. Unlike the ordinary umbrella, however, the hinges connecting the ribs to the hub in the described deployable antenna are somewhat complex structures, which need not be described in detail. The reason for that complexity is that the hinges must carry "launch loads" in the stowed position. Then they are required to deploy the rib to the correct angle so that the rib provides the correct shape for the reflective surface. Once deployed, the hinges must latch or be pre-loaded against mechanical stops to provide sufficient deployed stiffness. The foregoing requirements are satisfied by complexity of the elements, preciseness in shape, and difficulty in manufacturing and integration, which translates to a higher than desired manufacturing cost. As those skilled in the art appreciate, in the abstract the foregoing reflectors are somewhat expensive to manufacture, but serve their purpose well.
A recent trend in reflector use in sensor payloads, such as radars, requires reception of lower frequency ranges, namely P-band, between 400 and 440 MHz, specifically 425 MHz, and at higher UHF frequencies, L-band up to 1.8 GHz. Communications payloads also may operate at those frequency bands. On land, antennas for those frequencies are ordinarily served by the familiar Yagi antenna, containing a radiating element and multiple reflector elements spaced apart and oriented in parallel. An ordinary television antenna is an example of the Yagi type. However, it is also possible to use a parabolic antenna at those lower frequencies.
As those skilled in the art appreciate, designs of antenna structures, including those of the reflectors, may be mathematically scaled up in size, to provide deployable reflectors with improved gain. Unfortunately, scaling in size does not decrease the complexity or cost of the reflector or its hinges. Since these new applications are not critical to life or sovereignty, they command fewer investment dollars. Hence, those applications eschew the expensive existing deployable designs and, to achieve financial viability, desire a less expensive approach, which, until the present invention, was not available.
Accordingly, an object of the invention is to reduce the cost and complexity of manufacturing an off-set parabolic reflector antenna, particularly its deployable reflector.
A further object of the invention is to provide a new deployable reflector design that adapts and combines proven component designs found in other types of antennas to achieve higher reliability.
A still further object is to provide a deployable parabolic reflector that is particularly useful for transmission and collection of RF energy, particularly at P-band and in the UHF frequency regions, whose component elements are more simple in structure and less difficult to manufacture than existing deployable parabolic reflectors for like applications. And an ancillary object of the invention is to provide a like deployable parabolic reflector for concentrating light photons.